| The likely reason is the changes in lifestyle

Agreed, that's the commonly held reason but when one attempts to reverse what the changes seem to have been, by exercising more and eating less, that doesn't seem to work. My point is that perhaps we need to better understand what the changes have been, that perhaps they are more subtle than they are commonly held to be.

To couple on with what the other poster said there are two primary changes...

1. The amount of available cheap calories in convenient easy to eat packaging is orders of magnitudes higher than it was in the past.

2. Work and activity has changed so dramatically that we would not recognize the world of the past.

Now the nuance here is understanding all the small ramifications that are far more invisible to us. Think of a coke bottle and how many calories that contains, I mean it's a huge amount. But now think about the convenience of it. You can carry it with you just about anywhere. The vast majority of jobs will let you carry it around work or leave it on your desk to drink from all day. It doesn't matter how much your work out, if you drink one extra coke per day, you've added more calories in than you're working off.

But then add in all the small things that keep us from walking/exercising as much. Way fewer of our jobs are 'hard work' and been replaced by machines. We tend not to let our kids run around like wild creatures outside the house all day. Instead they are probably home, on the entertainment system of some kind with a coke in their hand. Kids also consume junk all day that is specifically marketed to them. You have to add all these factors in.

From what I understand (which is mostly second hand from a health journalist friend), "work out a bit more and eat a bit less" doesn't get you anywhere close to how people lived up until 1950. In 1920, Americans had 8million cars registered (at 106m people), today they have 276 million cars to 330m people, and that's basically the same for every aspect of life.